[Se]in and [Se]out : initial and final supernatant Se concentration

1. No association of Se0 with purified Boom Clay Humic Acid (p-BCHA) pH-pe diagram [Se]tot = 10-6 mol l-1 (NEA database project 2006) Se0 Se(IV) Se0 + p-BCHA + Fe(II); pellet Se0 + p-BCHA ; pellet Se(IV) + p-BCHA + N2H4;spn; pH 7.25 Se(IV) + p-BCHA + N2H4;spn; pH 5.25 Sample Description Solid/Liquid [Se]in (M) [Se]out (M) [Se]S (mol/kg) pH 3 FeS2 < 10 µm NPT 100 g·l-1 6.1010-3 4.8010-5 6.11×10-2 7.80 4 FeS2 < 10 µm PT 100 g·l-1 6.1010-3 8.63×10-4 5.25×10-2 7.42 • Sorption-Reduction by Fe(II) containing minerals (FeS2, FeS, …) * • Batch tests of SeO32‑ with pure pyrite (FeS2) in synthetic Boom Clay water (no OM) • Selenium on FeS2 solid phase (mol g‑1) versus SeO32‑ concentration (mol dm‑3). • Short contact times (3 and 7 days) : linear Kd  sorption process • Longer contact times : progressive convergeance to 3 × 10‑9 mol dm‑3  reduction process. Sorption-reduction of SeO32‑ on pyrite (FeS2) Sample Description Solid/Liquid [Se]in (M) [Se]out (M) [Se]S (mol/kg) pH TIME 9 FeS < 100 µm NPT 100 g·l-1 6.0810-3 < 10-6 > 6.10×10-2 7.89 10 FeS < 100 µm PT 100 g·l-1 6.0710-3 < 10-6 > 6.6810-2 mol/kg 7.86 • Sorption on oxidic surfaces (edges) of clay minerals SeO3-2 sorption on Illite du Puy (10 g/l; 0.01 N NaCl) • Association of Se with Boom Clay Organic Matter TIME State-of-the-art regarding selenium geochemistry under Boom Clay conditions Bruggeman, C.; Breynaert E.; Maes, A Katholieke Universiteit Leuven, Laboratory for Colloid Chemistry, Kasteelpark Arenberg 23, B-3001 Leuven, Belgium Se Redox Chemistry Se(0) • Oxidising Conditions • Selenate (SeO42-) • Weakly adsorbed on oxide surfaces • Reduction kinetically hindered / poorly known • No solubility limitation • Mildy Oxidising Conditions • Selenite (SeO32-) • Strongly adsorbed on oxide surfaces • Reduction faster than SeO4-2, important process for storage • No solubility limitation Se(IV) + p-BCHA + N2H4;pellet; pH 5.25 • Reduction of SeO3-2 to Se0 by pyrite (FeS2) • Reducing Conditions • Se0 , Selenide (Se2-) • Limited solubility • Not well known • [Se]in and [Se]out : initial and final supernatant Se concentration • PT and NPT: samples pretreated and not pretreated with 1M HCl after crushing • XAS analysis, Se K-edge (12.658 keV), fluorescence mode Se(VI) • No significant reduction or sorption by Boom Clay components : • Illite • Pyrite • HS Se(IV) Se(-II) • Formation of FeSe by reduction of SeO3-2 by FeS, 18 days equilibrium * Selenite reduction in Boom clay: Effect of FeS2, clay minerals and dissolved organic matter. Bruggeman C, Maes A, Vancluysen J and P. Vandemussele. ENVIRONMENTAL POLLUTION 137 (2): 209-221 • [Se]in and [Se]out : initial and final supernatant Se concentration • PT and NPT: samples pretreated and not pretreated with 1M HCl after crushing • XAS analysis, Se K-edge (12.658 keV), fluorescence mod • Short term (1 day) batch sorption edges of Se(IV) on illite du Puy • Typical anion adsorption edges • Conclusions • Under BK conditions • Se(VI)  unaltered • Se(IV)  initial sorption on oxidic surfaces  reduction by FeS2 • Knowledge Gaps ? •  Se(VI) sorption on Boom Clay  Se(IV) sorption/reduction on CaCO3/FeCO3phases  Se speciation on Boom Clay solid phase • Association increases with time  speciation/valence ? Acknowledgements: ONDRAF/NIRAS – TRANCOM II – FUNMIG – FWO/NWO